FIGS. 1A and 1B show the construction of a conventional microwave plasma processing apparatus 100 having such a radial line slot antenna, wherein FIG. 1A shows the microwave plasmas processing apparatus in a cross-sectional view while FIG. 1B shows the construction of the radial line slot antenna.
Referring to FIG. 1A, the microwave plasma processing apparatus 100 has a processing chamber 101 evacuated from plural evacuation ports 116, and there is formed a stage 115 for holding a substrate 114 to be processed. In order to realize uniform processing in the processing chamber 101, a ring-shaped space 101A is formed around the stage 115, and the plural evacuation ports 116 are formed in communication with the foregoing space 101A with a uniform interval, and hence in axial symmetry with regard to the substrate. Thereby, it becomes possible to evacuate the processing chamber 101 uniformly through the space 101A and the evacuation ports 116.
On the processing chamber 101, there is formed a plate 103 of plate-like form at the location corresponding to the substrate 114 on the stage 115 as a part of the outer wall of the processing chamber 101 via a seal ring 109, wherein the shower plate 103 is formed of a dielectric material of small loss and includes a large number of apertures 107. Further, a cover plate 102 also of a dielectric material of small loss is provided on the outer side of the shower plate 103 via another seal ring 108.
The shower plate 103 is formed with a passage 104 of a plasma gas on the top surface thereof, and each of the plural apertures 107 are formed in communication with the foregoing plasma gas passage 104. Further, there is formed a plasma gas supply passage 106 in the interior of the shower plate 103 in communication with a plasma gas supply port 105 provided on the outer wall of the processing vessel 101. Thus, the plasma gas of Ar, Kr or the like supplied to the foregoing plasma gas supply port 105 is supplied to the foregoing apertures 107 from the supply passage 106 via the passage 104 and is released into a space 103B right underneath the shower plate 103 in the processing vessel 101 from the apertures 107 with substantially uniform concentration.
On the processing vessel 101, there is provided a radial line slot antenna 110 having a radiation surface shown in FIG. 1B on the outer side of the cover plate 102 with a separation of 4–5 mm from the cover plate 102. The radial line slot antenna 110 is connected to an external microwave source (not shown) via a coaxial waveguide 110A and causes excitation of the plasma gas released into the space 101B by the microwave from the microwave source. It should be noted that the gap between the cover plate 102 and the radiation surface of the radial line slot antenna 110 is filled with the air.
The radial line slot antenna 110 is formed of a flat disk-like antenna body 110B connected to an outer waveguide of the coaxial waveguide 110A and a radiation plate 110C is provided on the mouth of the antenna body 110B, wherein the radiation plate 110C is formed with a number of slots 110a and slots 110b wherein slots 110b are formed in a direction crossing the slots 110a perpendicularly as represented in FIG. 1B. Further, a retardation plate 110D of a dielectric film of uniform thickness is inserted between the antenna body 110B and the radiation plate 11C.
In the radial line slot antenna 110 of such a construction, the microwave supplied from the coaxial waveguide 110 spreads between the disk-like antenna body 110B and the radiation plate 110C as it is propagated in the radial direction, wherein there occurs a compression of wavelength as a result of the action of the retardation plate 110D. Thus, by forming the slots 110a and 110b in concentric relationship in correspondence to the wavelength of the radially propagating microwave so as to cross perpendicularly with each other, it becomes possible to emit a plane wave having a circular polarization state in a direction substantially perpendicular to the radiation plate 110C.
By using such a radial line slot antenna 110, uniform plasma is formed in the space 101B right underneath the shower plate 103. The high-density plasma thus formed is characterized by a low electron temperature and thus, there is caused no damaging of the substrate 114 and there is caused no metal contamination as a result of the sputtering of the vessel wall of the processing vessel 101.
In the plasma processing apparatus of FIG. 1, it should further be noted that there is provided a conductive structure 111 in the processing vessel 101 between the shower plate 103 and the substrate 114, wherein the conductive structure is formed with a number of nozzles 113 supplied with a processing gas from an external processing gas source (not shown) via a processing gas passage 112 formed in the processing vessel 101, and each of the nozzles 113 releases the processing gas supplied thereto into a space 101C between the conductive structure 111 and the substrate 114. It should be noted that the conductive structure 111 is formed with openings between adjacent nozzles 113 with a size such that the plasma formed in the space 101B passes efficiently from the space 101B to the space 101C by way of diffusion.
Thus, in the case a processing gas is released into the space 101C from the conductive structure 111 via the nozzles 113, the processing gas is excited by the high-density plasma formed in the space 101B and a uniform plasma processing is conducted on the substrate 114 efficiently and with high rate, without damaging the substrate or the devices on the substrate, and without contaminating the substrate. Further, it should be noted that the microwave emitted from the radial line slot antenna is blocked by the conductive structure and there is no possibility of such a microwave causes damaging in the substrate 114.
Meanwhile, the density of the plasma formed in the space 101B can reach the order of 1012/cm3 in such a plasma processing apparatus 110 that uses the radial line slot antenna 110. Thus, the shower plate 103 is exposed to a large amount of ions and electrons constituting the high-density plasma, and the ions and electrons thus formed cause heating. The thermal flux caused by such ions and electrons can reach the value of as much as 1–2 W/cm2. In view of the fact that the plasma processing apparatus 100 is frequently operated by maintaining the wall temperature of the processing chamber 101 to about 150° C. so as to suppress formation of deposits on the processing chamber 101, there is caused accumulation of heat in the shower plate 103 and the cover plate 102 formed of a dielectric material, as a result of heating of the processing chamber 101. As a result, there is formed a very large temperature distribution.
FIG. 2 shows the temperature distribution formed in the shower plate 103 for the case the wall temperature of the processing chamber 101 is set to 150° C. and the heat of the high-density plasma formed in the space 101B has been transferred into the shower plate 103 with a flux of 1 W/cm2. Here, the thickness of the shower plate 103 is set to 25 mm.
Referring to FIG. 2, it can be seen that the temperature at the central part of the shower plate exceeds far beyond 600° C. in the case a quartz glass having thermal conductivity of 1.4 W/m·K is used for the shower plate 103. In view of the large thermal strain associated with the temperature difference, it is concluded that such a shower plate is not suitable for practical use. In the case the shower plate is formed of Al2O3 having thermal conductivity of 1.5 W/m·K, or in the case of an Al2O3 shower plate formed by a hot isostatic pressing (HIP) and having thermal conductivity of 30 W/m·K, too, the temperature at the central part of the shower plate becomes 450° C. or more or 300° C. or more, and a very large thermal strain is applied to the shower plate 103. In such a high temperature, there arises another problem that a gas of low decomposition temperature cannot be used for the plasma gas because of the decomposition.
In the case AlN is used for the shower plate 103, on the other hand, there occurs efficient dissipation of heat in the radiation direction because of the large thermal conductivity of 160 W/m·K, and the temperature rise at the central part of the shower plate 103 as a result of heat accumulation becomes minimum.
Because of this reason, it has been practiced to use AlN for the shower plate 103 and also for the cover plate in the plasma processing apparatus 100 of FIG. 1 that uses a radial line slot antenna.
However, AlN is a material of large dielectric loss, and the dielectric loss, represented in terms of tan δ takes the value of about 3×10−3. Thus, in the case the shower plate 103 and the cover plate 102 are formed of AlN, there is caused substantial loss in the microwave emitted by the antenna 110 and efficient excitation of plasma is not possible. In other words, the conventional plasma processing apparatus 100 of FIG. 1 has suffered from the problem, associated with the use of AlN for the shower plate 103 and the cover plate 102, in that efficiency of plasma excitation is not sufficient. As a result, it has been necessary to use a microwave source of large output power in the conventional plasma processing apparatus 100 and ignition of plasma has been difficult.